In order to provide a rubber product such as a transmission belt or a tire with tensile strength and dimensional stability, it is common practice to embed a high-strength fiber e.g. a glass fiber, a nylon fiber or a polyester fiber as a reinforcement in a base rubber of the rubber product. The rubber-reinforcing fiber, for use as the reinforcement embedded in the base rubber, needs to have good adhesion to the base rubber to define a tight interface between the rubber-reinforcing fiber and the base rubber and prevent separation of the rubber-reinforcing fiber from the base rubber. The glass fiber itself cannot however be adhered to the base rubber and, even if adhered, shows such weak adhesion as to cause interfacial separation between the glass fiber and the base rubber and fail to function properly as the reinforcement.
There is accordingly often used in e.g. the transmission belt a rubber-reinforcing glass fiber produced by preparing a glass-fiber coating liquid in which a resorcinol-formaldehyde resin and various latex components are dispersed in water, and then, applying and drying a coating layer of the glass-fiber coating liquid onto a glass fiber cord of filament yam, so as to enhance adhesion between the glass fiber and the base rubber and prevent interfacial separation of the glass fiber from the base rubber. The coating layer has the effect of adhering the glass fiber to the base rubber when the rubber-reinforcing glass fiber is embedded in the base rubber and formed into the transmission belt under high-temperature conditions, but the strength of adhesion between the glass fiber and the base rubber is not always sufficient. For example, a heat-resistant rubber such as hydrogenated nitrile rubber (hereinafter abbreviated as “HNBR”) is employed as the base rubber of the automotive transmission belt for use in a high-temperature engine room environment. In the case where the rubber-reinforcing glass fiber is treated only with the above coating process and embedded in the heat-resistant base rubber, however, the transmission belt cannot maintain adhesion strength between the rubber-reinforcing glass fiber and the base rubber during running where the transmission belt is continuously bent under high-temperature conditions. This can result in the occurrence of interfacial separation between the rubber-reinforcing glass fiber and the base rubber during long hours of running.
In view of the foregoing, Patent Documents 1 to 6 propose the production of rubber-reinforcing glass fibers for use in transmission belts, by performing the above coating process to form primary coating layers on glass fiber cords and applying and drying secondary coating liquids of different compositions to form secondary coating layers on the primary coating layers, such that the transmission belts become able to maintain adhesion of the rubber-reinforcing glass fibers to cross-linked HNBR belt materials, without causing interfacial separation between the rubber-reinforcing glass fibers and the cross-linked HNBR materials, and to secure long-term reliability even under high-temperature running conditions.
More specifically, Patent Document 1 discloses a coating treatment technique that uses a secondary coating liquid containing a halogen-containing polymer and an isocyanate.
Patent Document 2 discloses a rubber-reinforcing glass fiber cord produced by applying, drying and curing onto a rubber-reinforcing glass fiber a primary coating of treatment liquid containing a resorcinol-formaldehyde condensate and a rubber latex, and then, applying drying and curing a secondary coating of different treatment liquid onto the primary coating, wherein the secondary coating treatment liquid contains as main components a rubber-blended material, a vulcanization agent and a maleimide-based vulcanization accelerator.
Patent Document 3 discloses a rubber-reinforcing glass fiber cord produced by applying, drying and curing onto a rubber-reinforcing glass fiber a primary coating of treatment liquid containing a resorcinol-formaldehyde condensate and a rubber latex, and then, applying drying and curing a secondary coating of different treatment liquid onto the primary coating, wherein the secondary coating treatment liquid contains as main components a rubber-blended material, a vulcanization agent and a dimethacrylate-based vulcanization accelerator and the rubber-blended material is a mixed rubber solution of a hydrogenated nitrile rubber and a hydrogenated nitrile rubber in which zinc methacrylate is dispersed.
Patent Document 4 discloses a rubber-reinforcing fiber treatment liquid containing a rubber latex, a water-soluble resorcinol-formaldehyde condensate and a triazinethiol.
Patent Document 5, filed by the present applicant, discloses a rubber-reinforcing glass fiber material produced by applying and drying onto a glass fiber a coating of emulsified glass-fiber coating liquid in which an acrylic ester resin, a vinylpyridine-stylene-butadiene copolymer and a resorcinol-formaldehyde resin are dispersed in water, and then, applying another coating of glass-fiber coating liquid in which a halogen-containing polymer and 0.3 to 10.0% by weight of a bis-allylnagiimide, with respect to the weight of the halogen-containing polymer, are dispersed in an organic solvent. This rubber-reinforcing glass fiber material has been proven to show good adhesion to HNBR.
Patent Document 6, filed by the present applicant, discloses a rubber-reinforcing glass fiber material produced by applying, drying and curing onto a glass fiber a primary coating of glass-fiber coating liquid in which a resorcinol-formaldehyde resin and a rubber latex are dispersed in water, and then, applying, drying and curing onto the primary coating a secondary coating of different glass-fiber coating liquid in which a bis-allylnagiimide, a rubber elastomer, a vulcanization agent and an inorganic filler are dispersed in an organic solvent. This rubber-reinforcing glass fiber material has also been proven to show good adhesion to HNBR and, when embedded in HNBR for use as the reinforcement in the transmission belt, show high heat resistance without a deterioration in tensile strength even after long hours of running under high-temperature conditions.
As discussed above, the conventional heat-resistant transmission belt is produced by applying and drying the glass-fiber coating liquid of resorcinol-formaldehyde resin, vinylpyridine-stylene-butadiene copolymer and chlorosulfonated polyethylene onto the glass fiber cord and embedding the resulting rubber-reinforcing glass fiber in the heat-resistant HNBR material. Further, the rubber-reinforcing glass fiber is generally provided with the secondary coating layer before embedded in the heat-resistant HNBR material.    Patent Document 1: Japanese Examined Patent Publication No. 2-4715    Patent Document 2: Japanese Patent No. 3201330    Patent Document 3: Japanese Patent No. 3201331    Patent Document 4: Japanese Laid-Open Patent Publication No. 10-25665    Patent Document 5: Japanese Laid-Open Patent Publication No. 2004-203730    Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-244785